With the development of wireless network technology, by externally providing a plurality of co-frequency antennas on a device for wireless access (such as a wireless router) utilizing Multiple-Input Multiple-Output (MIMO) technology, the coverage and network connection speed of the device for wireless access can be improved using the signal superposition effect provided by the plurality of co-frequency antennas.
In the case of a wireless router, for example, in order to ensure that signals from the plurality of co-frequency antennas of the router do not interfere with one another, isolation of adjacent co-frequency antennas is required to be less than −25 dB. Typically, for a general-purposed design, a distance between adjacent co-frequency antennas of a router is required to be larger than one wavelength. For example, when the operating frequency of the co-frequency antennas is 2.45 GHz, one wavelength of the operating frequency in air medium is 122.25 mm. Thus, the distance between adjacent co-frequency antennas of the router is required to be larger than 122.25 mm.